Furosemide × Prednisolone

Both cause hypokalaemia: glucocorticoids via mineralocorticoid effect and muscle catabolism; furosemide via direct urinary potassium loss. Additive risk, especially on long-term prednisolone.

Weakness, muscle cramps, fatigue, palpitations, arrhythmia (particularly dangerous with digoxin). Symptoms appear at potassium below 3.5 mmol/L.

For short prednisolone courses (3–7 days), no specific adjustment needed. For long-term therapy, check potassium at 1, 2, and 4 weeks, then monthly. If it falls below 3.5 mmol/L, add potassium supplements (10–20 mmol/day) or a potassium-sparing diuretic. On digoxin, hypokalaemia is dangerous — arrhythmia risk.